Polybutadiene has a so-called microstructure in which a bonding portion formed by polymerization of 1, 4-position (1,4-structure) and a bonding portion formed by polymerization of 1,2-position (1,2-structure) are concurrently exist in a molecular chain. The 1,4-structure is classified into two types, namely cis structure and trans structure. On the other hand, the 1,2-structure has a vinyl group as a side chain.
It has been known that various polybutadienes having different microstructures are produced using a polymerization catalyst. Those polybutadienes are used for various purposes depending on their properties. In particular, polybutadiene having a high molecular linearity is excellent in wear resistance, heat resistance, and impact resilience. A ratio Tcp/ML1+4, which is a ratio of a viscosity of 5% toluene solution measured at 25° C. (Tcp) to Mooney viscosity measured at 100° C. (ML1+4), is used as a index of the linearity, Tcp represents a degree of entanglement of molecules in a dense solution. As Tcp/ML1+4 is larger, branching degree of polybutadiene is smaller and linearity of polybutadiene is larger.
As disclosed in Patent documents 1 to 3, it has been found that polybutadiene having the microstructure, which includes high cis structure, moderate amount of 1,2-structure, and small amount of trans structure, and thus having high linearity, can be produced by using a polymerization catalyst consisting of metallocene type complex of vanadium metal compound, ionic compound of non-coordinate anion and cation, and/or aluminoxane. It is considered that this polybutadiene is applied to a nigh-impact polystyrene resin and a tire because of its excellent properties. However, since a polymerization inhibitor does not speedily diffuse to the polybutadiene solution because of high viscosity of the polybutadiene solution induced by high linearity, deactivation of the polymerization catalyst is delayed to promote a side reaction of the polymerisation catalyst and antioxidant. As a result, chromogenic development of polybutadiene occurs and APHA color on a solution state becomes high. Since the rubber-reinforced polystyrene resin composition produced by using the polybutadiene occurs yellow discoloration when the APHA color is high, high APHA color is not preferred.
Further, since polybutadiene has a relatively high cold flow, there are some cases where the polybutadiene is demanded to improve on the occasion, of storage and transfer. For that reason, it has been found that the properties such as a cold flow can be improved by modifying the polybutadiene in the presence of a metal catalyst, in order to resolve the problems on the occasion of storage and transfer of polybutadiene having the microstructure which includes high cis structure, moderate amount, of 1,2-structure, and small amount of trans structure, and thus high linearity (Patent document 4).
On the other hand, acrylonitrile-butadiene-styrene copolymer (ABS polymer) obtained by radical polymerization of styrene monomer containing polybutadiene added thereto, is widely known as a high-impact polystyrene resin. For example, Patent document 5 discloses a high-impact polystyrene resin, in which high cis and high vinyl BR is used. The high cis and high vinyl BR has 65-95% of cis-1,4-structure and 4-30% of 1,2-structure and produced using a metallocene catalyst.
Furthermore, butadiene-styrene copolymer (HIPS polymer) obtained by radical polymerization of styrene monomer containing polybutadiene added thereto, is widely known as a high-impact polystyrene resin. For example, Patent documents 10-13 disclose a high-impact polystyrene resin, in which high cis and high vinyl BR is used, as a modifying agent for rubber. The high cis and high vinyl BR has 65-95% of cis-1,4-structure and 4-30% of 1,2-structure and produced using a metallocene catalyst. The rubbers described in these documents are modified to improve a cold flow property, and the obtained polymer is reported as HIPS polymer (Patent document 6).